Breathe and Breathe In

Breathe

Breathe is an action word meaning to bring air, or oxygen, into your lungs and then push it out.
EX: It is so crowded in here that I feel like I cannot breathe. If I can capture some air in my lungs, we can continue our conversation.
EX: The child struggling in the water was gulping for air as his head bobbed above the surface. When the lifeguard realised the child could not breathe, he jumped in to save him.
The popular phrase take a deep breath means that you need to calm down and take a minute to collect yourself, as if in anger or another emotional state. Don't hold your breath is a common phrase meaning that you should not wait for a specific outcome or rely on something happening.
EX: You need to take a deep breath before responding to that letter. You do not want the full weight of your wrath unleashing.
EX: I know you think the accused criminal will be indicted, but don't hold your breath. There may be evidence that will release him.

Breathe In

The phrase breathe in refers to the action of taking air or any substance into your lungs.
EX: When we put the mask on your face, breathe in. It is wise to let the gas gently put you to sleep before surgery.
EX: Try to breathe in slowly and not gulp the air. This will keep your heart rate down and help your body relax.
The word inhale is the scientific term and synonym for breathe in. Inhale is commonly used to talk about cigarette smoke. The verb to inhale can also mean to eat or drink something too quickly.
EX: Are you sure you really smoke? I have not seen you inhale, so I think you are just holding that cigarette for effect.
EX: Do not inhale your food! This is dangerous because you could choke and really hurt yourself.